The long-range goal of this project is to help determine the etiology and pathogenesis for as many tumor types and tumor sites as possible through the comparative approach of comparing naturally occurring cancers in various species including man. The study of the natural history of cancer in domestic animals and the interrelationship of cancer in man and animals has been limited because of the lack of adequate animal data. The Animal Neoplasm Registry in Alameda and Contra Costa Counties, California, initiated July, 1963, has developed methods of systematically collecting cancer cases diagnosed by practicing veterinarians. This is the first general animal population-based registry in the world of histopathologically confirmed tumors of all types diagnosed in the animal populations of a defined geographic area. A population based human tumor registry also functions in the same area. Thus, human and animal cancer cases are available for comparision from the same geographic area. From these data, the following types of studies can be done: (1) characterization of animal cancers such as incidence rates, including age, sex- and breed-specific, (2) etiological studies such as seeking factors or variables that may be affecting the risk or occurrence of various cancers in animals and relating them to human cancer occurrence, (3) human-animal association studies such as whether there is increased human cancer in households where an animal with cancer has lived, (4) human-animal comparisions such as age- and sex-specific incidence rate patterns for clues to possible environmental effects modifying patterns, (5) laboratory studies such as attempts to isolate oncogenic viruses from various tumors or other cancerous animal tissues, and (6) treatment evaluation studies such as evaluation of new treatment methods or approaches prior to application to man.